This invention relates to apparatus for and methods of measuring the thickness of materials by the use of ultrasonic techniques.
It is the conventional practice, when making ultrasonic measurements of the thickness of materials in the range from 1 mm upwards, to use a dual probe where the transmitter and receiver crystals of piezoelectric material are separated, and where each of the transmitter and receiver elements has an acoustic "stand-off" between its piezoelectric crystal and the actual working surface of the probe. When carrying out measurements at room temperature it is usual to use plastics materials for the stand-off, while for measurements at higher temperatures one can use a polyamide material, quartz or certain glass materials. Ultrasonic probes for thickness measurement may be required to function at different frequencies, and both the size and shape of the probes will depend inter alia upon the frequency of excitation and the particular application for which the probe is to be used. For example, in the case of a probe designed to measure the thickness of high temperature materials, it may be necessary to provide a relatively long stand-off in order to ensure that there is sufficient thermal insulation between the piezoelectric crystal material and the working surface of the probe which contacts the material under test, since most piezoelectric crystal materials have a Curie point of less than 300.degree. C.
In making a measurement of the thickness of a piece of material, because the ultrasonic pulses have to traverse both the stand-off and the material under test, it is necessary to subtract from the time interval between the excitation of the transmitter crystal and the reception of the first echo from the far side of the material under test a time period which is equal to the transmission time of the ultrasonic pulses through the material of the stand-off. This subtraction results in a time period which is representative of the actual thickness of the material under test; this elapsed time figure can readily be converted into a thickness figure.
One conventional method of performing this subtraction is by including in the instrument a monostable circuit which is triggered at the instant that the transmitter crystal is excited and which has a monostable period which is equal to the time taken by the ultrasonic pulses to traverse the material of the stand-off. One can make the monostable time period fully adjustable by providing a multi-turn control knob. Alternatively, and more usually, the monostable time period can be preset by the operator by the use of a control device which is adjustable by means of a screwdriver. The latter type of adjustment normally only provides a "fine trim" about a predetermined value which is set by the internal components of the instrument.
Measuring instruments which incorporate such means of adjustment can, as a consequence, only use probes which have stand-off times which fall within a fairly narrow range. Particularly where a screwdriver "fine" adjustment is provided one is not normally able to use more than one or two types of probe with the measuring instrument. If a zero control is fitted, the control can be made to cover a wide range with adequate resolution by making the control a multi-turn adjustment element. This does enable one to use a wider range of probes with the measuring equipment, but such an adjustment places much more onus on the operator to make the right settings. The wider the range of adjustment permitted by the control means, the larger is the possible error if the adjustment setting is incorrect.
In certain known measuring instruments a time representative of the acoustic stand-off time is preset inside the instrument and there is no provision for the operator to make a zero adjustment of the probe. However, a preset fixed delay time of this nature is only able to give moderate accuracy for a limited period because in practice the face of the probe is normally subject to wear as it moves on the workpiece surface, thus causing a change in the actual acoustic transit time through the stand-off material. Furthermore, in the case of high temperature measurements, the temperature of the stand-off material will increase as a series of measurements are carried out, and this will produce a change in the ultrasonic velocity through the stand-off material and hence a change in the acoustic delay time. An instrument with a preset delay time can obviously not cater for such variations, and even with the provision of a manual zero adjustment control it is extremely difficult to make any reliable compensation for such temperature changes, primarily because by the time that the zero control has been reset the temperature of the stand-off material will probably have decreased, because the probe normally has to be removed from the workpiece during the adjustment process.
In some known instruments which provide for manual adjustment of the stand-off delay time it is possible to "offset" the instrument so that the effective readings start at some thickness other than zero, provided that the range covered by the zero control is adequate. For example, one might adjust the zero so that readings displayed on the meter of the instrument start from one inch and go upwards, so that a 11/2 inch workpiece would then read 0.5 on the meter scale instead of 1.5. Hoever, such an arrangement has a number of disadvantages. In many cases it would not then be possible to take any readings below a certain minimum thickness and the arrangement is much more subject to operator error due to inability to make the proper adjustment or failure to interpret the readings correctly.